When using the "Line and stacked column chart" whenever the column value is blank the line values are ignored and don't contribute to the line chart, so that the line connects only data points over non-blank columns. This seems like a bug.
The work-around is to activate "Show items with no data", but that has other undesirable side-effects, like showing all of the shared axis even if there is no data at all (eg if shared axis is a Date column from a Calendar table covering a much wider time span).
(BTW, I was looking quite extensively for the best way to report bugs for Power BI (Desktop) and ended up here. Why do I have to submit this as a "New Idea"? ...and with completely unapplicable "Idea Labels"? A straight-foward way to report what's perceived as a bug would be highley appreciated!)